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Abstract:

A microphone system has a base coupled with first and second microphone
apparatuses. The first microphone apparatus is capable of producing a
first output signal having a noise component, while the second microphone
apparatus is capable of producing a second output signal. The first
microphone apparatus may have a first back-side cavity and the second
microphone may have a second back-side cavity. The first and second
back-side cavities may be fluidly unconnected. The system also has
combining logic operatively coupled with the first microphone apparatus
and the second microphone apparatus. The combining logic uses the second
output signal to remove at least a portion of the noise component from
the first output signal.

Claims:

1. A microphone system comprising: a base; a first microphone apparatus
coupled to the base, the first microphone apparatus capable of producing
a first output signal having a noise component, the first microphone
apparatus having a first back-side cavity; a second microphone apparatus
coupled to the base, the second microphone apparatus capable of producing
a second output signal, the second microphone apparatus having a second
back-side cavity, the first and second back-side cavities being fluidly
unconnected; and combining logic operatively coupled with the first
microphone apparatus and the second microphone apparatus, the combining
logic using the second output signal to remove at least a portion of the
noise component from the first output signal.

2. The microphone system as defined by claim 1 wherein the second output
signal comprises data relating to the mechanical response of the first
microphone apparatus.

3. The microphone system as defined by claim 1 wherein the second
microphone apparatus has a diaphragm and a cap acoustically sealing the
diaphragm.

4. The microphone system as defined by claim 1 wherein the second
microphone apparatus comprises a microphone and a low pass filter.

5. The microphone system as defined by claim 1 wherein the first
microphone apparatus comprises a first microphone, the second microphone
apparatus comprising a second microphone, the second microphone being
configured to have a low frequency cut-off that is greater than the low
frequency cut-off of the first microphone.

6. The microphone system as defined by claim 5 wherein the first
microphone has a first diaphragm and a first circumferential gap defined
at least in part by the first diaphragm, the second microphone having a
second diaphragm and a second circumferential gap defined at least in
part by the second diaphragm, the second circumferential gap being
greater than the first circumferential gap.

7. The microphone system as defined by claim 1 wherein the first
microphone apparatus has a first microphone having first diaphragm, the
second microphone apparatus having a second microphone with a second
diaphragm, the first and second diaphragms being exposed to a common
space.

8. The microphone system as defined by claim 1 wherein the second
microphone apparatus includes a microphone and a signal transformation
module.

9. The microphone as defined by claim 1 wherein the first microphone
apparatus comprises a first microphone, the second microphone apparatus
comprises a second microphone, the first microphone and second
microphones having different air leakage rates for providing the first
and second microphones with different low frequency cut-off points.

10. A microphone system comprising: a base; a first microphone apparatus
coupled with the base and capable of producing a first output signal, the
first microphone apparatus including a first microphone having a first
mechanical response and a first back-side cavity; a second microphone
apparatus coupled with the base and capable of producing a second output
signal, the second microphone apparatus including a second microphone
having a second mechanical response and a second back-side cavity, and
the first and second back-side cavities being fluidly unconnected; and
combining logic operatively coupled with the first microphone apparatus
and the second microphone apparatus, the combining logic combining the
first and second output signals to produce an output audio signal, the
first and second mechanical responses being effectively the same.

11. The microphone system as defined by claim 10 wherein the combining
logic includes a subtractor that subtracts the second output signal from
the first output signal.

12. The microphone system as defined by claim 10 further comprising means
for removing audible response of mechanical shock from the first output
signal.

13. The microphone system as defined by claim 10 wherein the second
microphone apparatus comprises an adaptive filter.

14. The microphone system as defined by claim 10 wherein the first output
signal comprises an audio component and a first noise component, the
second output signal comprising a second noise component, the combining
logic using the second noise component to mitigate the first noise
component from the first output signal to produce the output audio
signal.

15. A method of producing an output audio signal from a microphone
system, the method comprising: providing a base having a first microphone
for generating a first microphone output signal in response to an input
audio signal and a mechanical signal, the first microphone output signal
having an audio component and a mechanical component, the base also
having a second microphone for generating a second microphone output
signal in response to the mechanical signal, the first and second
microphones having back-side cavities that are fluidly unconnected to one
another; and using information from the second microphone output signal
to remove at least a portion of the mechanical component from the first
microphone output signal.

16. The method as defined by claim 15 wherein the second microphone
output signal has a second audio component, using comprising removing at
least a portion of the second audio component from the second microphone
output signal.

17. The method as defined by claim 15 further comprising fixed or
adaptively filtering the second microphone output signal.

18. The method as defined by claim 17 wherein the second microphone
output signal has a second audio component, using comprising removing at
least a portion of the second audio component from the second microphone
output signal before filtering.

19. The method as defined by claim 15 wherein the first microphone has a
first diaphragm that defines a first circumferential gap, the second
microphone having a second diaphragm that defines a second
circumferential gap, the second circumferential gap being greater than
the first circumferential gap.

20. The method as defined by claim 15 wherein the first microphone has a
first diaphragm, the second microphone having a second diaphragm, the
method exposing the first and second diaphragms to the input audio
signal.

Description:

PRIORITY

[0001] This patent application is a continuation of utility patent
application Ser. No. 12/546,073, entitled, "Noise Mitigating Microphone
System and Method," filed on Aug. 24, 2009, assigned attorney docket
number 2550/C45, and naming Kieran P. Harney, Jason Weigold, and Gary
Elko as inventors, the disclosure of which is incorporated herein, in its
entirety, by reference, which is a continuation in part of utility patent
application Ser. No. 11/492,314 entitled "Noise Mitigating Microphone
System and Method " filed Jul. 25, 2006, assigned attorney docket number
2550/B16, and naming Kieran P. Harney, Jason Weigold, and Gary Elko as
inventors, the disclosure of which is incorporated herein, in its
entirety, by reference.

[0003] The invention generally relates to microphones and, more
particularly, the invention relates to improving the performance of
microphones.

BACKGROUND OF THE INVENTION

[0004] Condenser microphones typically have a diaphragm that forms a
capacitor with an underlying backplate. Receipt of an audible signal
causes the diaphragm to vibrate to form a variable capacitance signal
representing the audible signal. It is this variable capacitance signal
that can be amplified, recorded, or otherwise transmitted to another
electronic device.

[0005] Problems arise, however, when the microphone is subjected to a
mechanical shock. Specifically, mechanical shocks can cause the diaphragm
to vibrate in a manner that degrades the microphone output signal.

SUMMARY OF THE INVENTION

[0006] In accordance with one embodiment of the invention, a microphone
system has a base coupled with first and second microphone apparatuses.
The first microphone apparatus is capable of producing a first output
signal having a noise component, while the second microphone apparatus is
capable of producing a second output signal. The first microphone
apparatus may have a first back-side cavity and the second microphone may
have a second back-side cavity. The first and second back-side cavities
may be fluidly unconnected. The system also has combining logic
operatively coupled with the first microphone apparatus and the second
microphone apparatus. The combining logic uses the second output signal
to remove at least a portion of the noise component from the first output
signal.

[0007] The second output signal may have, among other things, data
relating to the mechanical response of the first microphone apparatus.
Moreover, the second microphone apparatus may have a diaphragm and a cap
acoustically sealing the diaphragm. Alternatively, the diaphragm may be
exposed to a space to which another diaphragm in the system is exposed.
In some embodiments, the second microphone apparatus has a microphone and
a low pass filter.

[0008] Various embodiments of the first microphone apparatus have a first
microphone, while the second microphone apparatus has a second
microphone. The second microphone may be configured to have a low
frequency cut-off that is greater than the low frequency cut-off of the
first microphone. In addition, the first microphone may have a first
diaphragm and a first circumferential gap defined at least in part by the
first diaphragm, while the second microphone may have a second diaphragm
and a second circumferential gap defined at least in part by the second
diaphragm. The second circumferential gap illustratively is greater than
the first circumferential gap. This second gap effectively mitigates low
frequency audio components while the filter, if used, substantially
removes or mitigates remaining audio component.

[0009] To remove at least a portion of the noise components produced by
mechanical shock, the second microphone apparatus may have a microphone
and a signal transformation module (e.g., an adaptive filter).

[0010] In accordance with another embodiment of the invention, a
microphone system has a base coupled with first and second microphone
apparatuses. The first microphone apparatus is capable of producing a
first output signal and has a first microphone with a first mechanical
response. In a similar manner, the second microphone apparatus is capable
of producing a second output signal and has a second microphone with a
second mechanical response. The first microphone apparatus may have a
first back-side cavity and the second microphone may have a s back-side
cavity. The first and second back-side cavities may be fluidly
unconnected. The system also has combining logic operatively coupled with
the first and second microphone apparatuses. The combining logic combines
the first and second output signals to produce an output audio signal.
The first and second mechanical responses illustratively are effectively
the same.

[0011] Among other things, the combining logic may include a subtractor
that subtracts the second output signal from the first output signal. In
other embodiments, the combining logic may have an adder.

[0012] In accordance with another embodiment of the invention, a method of
producing an output audio signal from a microphone system provides a base
having a first microphone for generating a first microphone output signal
(having an audio component and a mechanical component) in response to an
input audio signal and a mechanical signal. The base also has a second
microphone for generating a second microphone output signal in response
to the mechanical signal. The first and second microphones may have
back-side cavities that are fluidly unconnected to one another. The
method uses information from the second microphone output signal to
remove at least a portion of the mechanical component from the first
microphone output signal.

[0013] The second microphone output signal may have a second audio
component. In that case, the method may remove at least a portion of the
second audio component from the second microphone output signal. In
addition, the method may adaptively filter the second microphone output
signal. Among other ways, the method may remove at least a portion of the
second audio component from the second microphone output signal before
adaptively filtering.

[0014] Illustrative embodiments of the invention are implemented as a
computer program product having a computer usable medium with computer
readable program code thereon. The computer readable code may be read and
utilized by a computer system in accordance with conventional processes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing advantages of the invention will be appreciated more
fully from the following further description thereof with reference to
the accompanying drawings wherein:

[0016]FIG. 1 schematically shows a base having a microphone system
configured in accordance with illustrative embodiments of the invention.

[0017] FIG. 2 schematically shows a cross-sectional view of a MEMS
microphone that may be used with illustrative embodiments of the
invention.

[0018] FIG. 3A schematically shows a plan view of the microphone system in
accordance with a first embodiment of the invention.

[0019]FIG. 3B schematically shows a plan view of the microphone system in
accordance with a second embodiment of the invention.

[0020]FIG. 3c schematically shows a cross-sectional view of the
microphone system shown in 3A, in accordance with illustrative
embodiments of the present invention.

[0021]FIG. 3D schematically shows a cross-sectional view of an
alternative microphone system with a lid, in accordance with embodiments
of the present invention.

[0022] FIG. 3E schematically shows a cross-sectional view of an
alternative microphone system made on a single die, in accordance with
further embodiments of the present invention.

[0023]FIG. 3F schematically shows a cross-sectional view of an additional
alternative microphone system in accordance with further embodiments of
the present invention.

[0024]FIG. 4A schematically shows the frequency response for the primary
microphone in the microphone system of illustrative embodiments of the
invention.

[0025]FIG. 4B schematically shows the frequency response for the
correction microphone in the microphone system of illustrative
embodiments of the invention.

[0027]FIG. 6 shows a process used by the microphone system of FIG. 1 to
produce an audible signal in accordance with illustrative embodiments of
the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] In illustrative embodiments, a microphone system has a primary
microphone and a correction microphone coupled to the same base to both
receive the same noise signals (e.g., mechanical shock signals) and react
in a corresponding manner. To improve the quality of the output audio
signal it produces, the microphone system uses noise signals detected by
the correction microphone to remove significant amounts of noise from the
signal produced by the primary microphone. As a result, the output audio
signal should have less noise than if not processed and noise is present.
Details of illustrative embodiments are discussed below.

[0029]FIG. 1 schematically shows a mobile telephone acting as a base 10
for supporting a microphone system 12 configured in accordance with
illustrative embodiments of the invention. To that end, the mobile
telephone (also identified by reference number 10) has a plastic body 14
containing the microphone system 12 for producing an output audio signal,
an earpiece 16, and various other components, such as a keypad,
transponder logic and other logic elements (not shown). As discussed in
greater detail below, the microphone system 12 has a primary microphone
18A and a correction microphone 18B that are both fixedly secured in very
close proximity to each other, and fixedly secured to the telephone body
14. More generally, both microphones 18A and 18B illustratively are
mechanically coupled to each other (e.g., via the base 10 or a direct
connection) to ensure that they receive substantially the same mechanical
signals. For example, if the telephone 10 is dropped to the ground, both
microphones 18A and 18B should receive substantially identical
mechanical/inertial signals representing the movement and subsequent
shock(s) (e.g., if the telephone 10 bounces several times after striking
the ground) of the telephone 10.

[0030] In alternative embodiments, the microphone system 12 is not fixedly
secured to the telephone body 14--it may be movably secured to the
telephone body 14. Since they are mechanically coupled, both microphones
18A and 18B nevertheless still should receive substantially the same
mechanical signals as discussed above. For example, the two microphones
18A and 18B may be formed on a single die that is movably connected to
the telephone body 14. Alternatively, the microphones 18A and 18B may be
formed by separate dies packaged together or separately.

[0031] The base 10 may be any structure that can be adapted to use a
microphone. Those skilled in the art thus should understand that other
structures may be used as a base 10, and that the mobile telephone 10 is
discussed for illustrative purposes only. For example, among other
things, the base 10 may be a movable or relatively small device, such as
the dashboard of an automobile, a computer monitor, a video recorder, a
camcorder, or a tape recorder. The base 10 also may be a surface, such as
the substrate of a single chip or die, or the die attach pad of a
package. Conversely, the base 10 also may be a large or relatively
unmovable structure, such as a building (e.g., next to the doorbell of a
house).

[0032] FIG. 2 schematically shows a cross-sectional view of a MEMS
microphone (identified by reference number 18) generally representing the
structure of one embodiment of the primary and correction microphones 18A
and 18B. Among other things, the microphone 18 includes a static
backplate 22 that supports and forms a capacitor with a flexible
diaphragm 24. In illustrative embodiments, the backplate 22 is formed
from single crystal silicon, while the diaphragm 24 is formed from
deposited polysilicon. A plurality of springs 26 (not shown well in FIG.
2, but more explicitly shown in FIGS. 3A and 3B) movably connect the
diaphragm 24 to the backplate 22 by means of various other layers, such
as an oxide layer 28. To facilitate operation, the backplate 22 has a
plurality of throughholes 30 that lead to a back-side cavity 32.
Depending on the embodiment and its function, the microphone 18 may have
a cap 34.

[0033] Audio signals cause the diaphragm 24 to vibrate, thus producing a
changing capacitance. On-chip or off-chip circuitry (not shown) converts
this changing capacitance into electrical signals that can be further
processed. It should be noted that discussion of the microphone of FIG. 2
is for illustrative purposes only. Other MEMS or non-MEMS microphones
thus may be used with illustrative embodiments of the invention.

[0034] One function of the primary microphone 18A is to produce a primary
signal having an audio component and a (zero or non-zero) noise
component. This noise component can include, among other things, 1) a
mechanical portion and 2) audio responses to the mechanical portion of
the noise component. For example, the mechanical portion of the noise
component could be the response of the microphone when it is dropped to
the ground (i.e., its diaphragm 24 moves as an inertial response). As
another example, the audio response to the mechanical portion of the
noise signal may be the initial sound and resultant of echoes generated
when the microphone/base 10 strikes the ground. Alternatively, the
mechanical portion of the noise component could be the response of the
microphone to wind (e.g., wind entering the mouthpiece of the telephone).
The primary microphone 18A also may be packaged or capped, as shown,
(e.g., a post-processing cap or in-situ cap) with a through-hole to
permit ingress of audio signals.

[0035] One function of the correction microphone 18B is to generate a
correction signal that can be used to substantially mitigate much of the
noise component of the primary signal. This mitigation may remove a
significant portion, or a relatively small portion, of the noise
component of the primary signal. Various embodiments, however, preferably
remove substantially all of the discussed noise components. Removing the
noise component should enhance the quality (e.g., the signal to noise
ratio) of the ultimate output signal.

[0036] The overall amount and type of mitigation may depend on the
application. For example, some embodiments remove the mechanical portion
of a noise component only. Other embodiments remove both the mechanical
portion and its audio response. Yet other embodiments may remove the
audio response portion of the noise signal only.

[0037] The correction microphone 18B may be considered to act as an
effective accelerometer within the microphone system 12. Accordingly, in
this context, the term "microphone" may be used generally to include
other devices, such as inertial sensors. Regardless its exact name, the
correction microphone 18B assists in mitigating inertial based noise
(i.e., signals causing undesired diaphragm displacement and related
noise). In some embodiments, rather than using the discussed correction
microphone 18B, the microphone system 12 thus has an accelerometer, such
as one or more one, two, or three axis IMEMS accelerometers produced and
distributed by Analog Devices, Inc. of Norwood, Mass.

[0038] The primary microphone 18A and correction microphone 18B preferably
are formed to have substantially identical responses to audio and noise
signals discussed herein. To that end, illustrative embodiments produce
two microphones 18A and 18B using substantially identical fabrication
processes and materials (e.g., silicon-on-insulator technology, or
conventional non-SOI surface micromachining processes that deposit layers
on a silicon wafer substrate). Accordingly, to the extent they can as
consistent with various discussed embodiments, the microphones 18A and
18B should have substantially identical diaphragm masses, backplates,
hole sizes, material, etc. . . . Alternative embodiments, however, may
use different microphones 18A and 18B that are calibrated to perform the
functions discussed herein.

[0039] As discussed in greater detail below with regard to FIG. 6,
illustrative embodiments combine the correction signal with the primary
signal to remove the noise component from the primary signal. Among other
ways, illustrative embodiments may subtract the correction signal from
the primary signal. Accordingly, to avoid subtracting the intended audio
signal from the primary signal, illustrative embodiments of the
correction signal substantially do not include the noted audio component
(e.g., it may include a significantly mitigated version of the audio
component). If the correction signal substantially has the audio
component, it would undesirably cancel or otherwise substantially
mitigate the audio component from the primary signal, thus substantially
undercutting one advantage of various embodiments of the system.

[0040] Various embodiments therefore physically shield the correction
microphone 18B from the input audio signal. In so doing, the correction
microphone diaphragm 24 receives mechanical (or related) signals, but
does not receive the audio signal. To physically shield the diaphragm 24,
the correction microphone 18B may 1) have a cap 34 that provides an
acoustic seal (i.e., shielding the correction microphone diaphragm 24) to
the diaphragm 24, 2) be contained within a sealed package, or 3) have
some other physical means for preventing the input audio signal from
contacting its diaphragm 24.

[0041] Other embodiments, however, logically shield the diaphragm 24 of
the correction microphone 18B from the input audio signal. If that
diaphragm 24 is logically shielded, then the diaphragms 24 of both of the
correction microphone 18B and the primary microphone 18A may be exposed
to a common space (e.g., the space through which the desired audio signal
traverses). In other words, both diaphragms 24 may receive essentially
the same audio input signal. FIGS. 3A and 3B schematically show two
embodiments that provide this functionality.

[0042] FIG. 3A schematically shows a plan view of the microphone system 12
in accordance with a first embodiment that logically shields the
correction microphone diaphragm 24. Specifically, the microphone system
12 includes the primary and correction microphones 18A and 18B fixedly
secured to an underlying printed circuit board 36, and logic 38 (see FIG.
5) for improving the quality of audio signals received by the primary
microphone 18A. Because it is a plan view, FIG. 3A shows the respective
diaphragms 24 of the microphones 18 and 18B and their springs 26. This
configuration of having a diaphragm 24 supported by discrete springs 26
produces a gap between the outer parameter of the diaphragm 24 and the
inner parameter of the structure to which each spring 26 connects. This
gap is identified in FIG. 3A as "gap 1" for the primary microphone 18A,
and "gap 2" for the correction microphone 18B.

[0043] As mentioned above, and as shown in FIG. 3c, in some embodiments,
the primary (e.g., 18A) and correction microphones (e.g., 18B) may be
formed on or secured to a single substrate or base, for example, the
circuit board 36. To that end, the circuit board 36 may be used to
segment or isolate the back-side cavities 32A and 32B of the primary
microphone 18A and the correction microphone 18B from one another. In
other words, in some embodiments, the back-side cavities 32A/32B may be
fluidly disconnected from one another (e.g., they do not share a common
volume; the back-side cavities are not fluidly connected). In this
manner, the primary microphone 18A and the correction microphone 18B may
by exposed to the same audio signal without interference from one another
(e.g., the motion of the diaphragms 24, changes in pressure within the
back-side cavities 32A/32B, etc.).

[0044] In addition to the segmented back-side cavities 32A/32B mentioned
above, one or both of the microphones 18A/18B may also have caps (e.g.,
caps 34A and 34B, respectively). For example, the primary microphone 18A
may have a cap 34A with an opening to allow the audio signal to reach the
diaphragm 24, and the correction microphone 18B may have a closed cap 34B
(e.g., with no opening) to physically shield the microphone 18B and/or
diaphragm 24 from the audio signal. Alternatively, both the primary
microphone 18A and the correction microphone 18B may have caps 34A with
openings to allow the audio signal to reach the diaphragms 24. In such
embodiments, the correction microphone 18B may be logically shielded as
described above.

[0045] As shown in FIG. 3D, some embodiments of the present invention may
also include a lid 320 that spans and covers the microphones 18A and 18B,
with segmented back volumes, to seal the microphones 18A/18B as a single
package. The lid 320 may have one or more openings 322 that allow the
audio signal to reach the primary microphone 18A. In such embodiments,
the microphones 18A and 18B may or may not also have the caps 34A/34B
described above (e.g., the primary microphone may have no cap or an open
cap 34A and the correction microphone may have either an open cap 34A, a
closed cap 34B, or no cap).

[0046] In some embodiments (e.g., those having no cap or an open cap on
the correction microphone 18B), the system may have a divider 340 located
between the primary microphone 18A and the correction microphone 18B.
This divider 340 physically shields the correction microphone 18B from
the audio signal entering through the opening 322 in the lid 320.
Alternatively, if the correction microphone 18B is not physically
shielded (e.g., there is no divider 340 and/or the correction microphone
18B has no cap or a cap with an opening), the correction microphone 18B
may be logically shielded as discussed above.

[0047] Although the microphones are described above as being separate
dies, the microphones may also be formed on the same die and maintain
segmented volumes, as shown in FIG. 3E. In such embodiments, the
microphones 330A/330B may be formed on or otherwise secured to a package
base 310, such as an FR-4 base. The package base 310 may then, in turn,
be secured to the telephone or the circuit board 36 discussed above. Like
the embodiments described above, embodiments formed on the same die may
also include caps 34A/34B. For example, the primary microphone 18A may
have an open cap 34A and the secondary microphone may have a closed cap
34B.

[0048]FIG. 3F shows a cross-sectional view of an additional embodiment of
the present invention, in which the audio signal enters the primary
microphone 18A through the bottom of the microphone 18A. To that end, the
substrate 36 may have an opening 360 (a "bottom port") to the back-side
cavity 32A, which effectively becomes a front volume. The area behind the
diaphragm now serves the function of a back volume. The opening 360 thus
allows the audio signal to enter what formerly acted as a back volume,
the back-side cavity 32A, and interact with the diaphragm 24 (e.g., cause
the diaphragm to vibrate), as described above.

[0049] Additionally, the substrate 36 may also have a similar opening (not
shown) for the correction microphone 18B. If the substrate 36 does not
have an opening for the correction microphone 18B, the correction
microphone will be physically shielded from the audio signal (e.g., if
the lid 350B described below does not have an opening). If the substrate
36 does have an opening for the correction microphone 18B, the correction
microphone may be logically shielded, as discussed above.

[0050] The primary and correction microphones 18A/18B of this embodiment
may also have lids 350A and 350B. In a manner similar to the caps 34A/34B
and lid 320 discussed above, the lids 350A/350B may or may not have
openings to allow the audio signal to reach the microphones 18A/18B from
the top. For example, the primary microphone lid 350A may have an opening
and the correction microphone lid 350B may not. Alternatively, the
correction microphone lid 350B may have an opening and the primary
microphone lid 350A may not, or both may have or not have an opening.

[0051] As mentioned in greater detail below and as shown in FIGS. 4A and
4B, some embodiments of the present invention can utilize microphones
having different low frequency cut-offs. It is important to note that
segmented and/or fluidly unconnected back-side cavities aid in creating
the different low frequency cut-offs shown in FIGS. 4A and 4B. In
particular, the segmented and/or fluidly unconnected back-side cavities
may allow each of the microphones 18A/18B to have separate and
independent air leakage rates past the diaphragms 24, as described in
greater detail below.

[0052] As known by those skilled in the art, it is generally desirable to
minimize the size of that gap (e.g., gap 1) to ensure that the microphone
can respond to low-frequency audio signals. In other words, if the gap is
too large, the microphone may not be capable of detecting audio signals
having relatively low frequencies. Specifically, with respect to the
frequency response of a microphone, the location of its low frequency
cut-off (e.g., the 3 dB point) is a function of this gap. FIG. 4A
schematically shows an illustrative frequency response curve of the
primary microphone 18A when configured in accordance with illustrative
embodiments of the invention. As shown, the low frequency cut-off is F1,
which preferably is a relatively low frequency (e.g., 50-100 Hz, produced
by an appropriately sized gap, such as a gap of about 1 micron).

[0053] It should be noted that this gap is anticipated to have no greater
than a negligible impact on the inertial response of the microphone to a
mechanical signal. Accordingly, although the microphone substantially
does not detect audio signals having frequencies below the low frequency
cut-off, it still can detect low-frequency inertial signals. For example,
a microphone having a gap sized to produce a low frequency cut-off of
approximately 350 Hz still should detect a mechanical signal having a
frequency of 150 Hz.

[0054] In accordance with one embodiment of the invention, gap 2 (of the
correction microphone 18B) is larger than gap 1 (of the primary
microphone 18A). Accordingly, as shown in FIG. 4B (showing the frequency
response of the correction microphone 18B), the low frequency cut-off F2
(e.g., 2-2.5 KHz, produced by an appropriately sized gap, such as about
5-10 microns) of the correction microphone 18B is much higher than the
low frequency cut-off F1 of the primary microphone 18A. As a result, the
correction microphone 18B does not adequately detect a wider range of
low-frequency audio signals. In other words, increasing the size of gap 2
effectively acts as an audio high pass filter for the correction
microphone 18B. As discussed in greater detail below, illustrative
embodiments use this effective high pass filter in combination with a
subsequent low pass filter 46 to significantly mitigate the response of
the correction microphone 18B to an input audio signal. Accordingly, the
correction microphone 18B does not require some means to shield it from
an input audio signal (e.g., a cap 34).

[0055] There are various ways to make gap 2 larger than gap 1 while still
ensuring that both microphones 18A and 18B have substantially identical
responses to noise signals. Among other ways, the diaphragms 24 may be
formed to have substantially identical masses. To that end, the diaphragm
24 of the correction microphone 18B may be thicker than the diaphragm 24
of the primary microphone 18A, while the diameter of the diaphragm 24 of
the correction microphone 18B is smaller than the diameter of the
diaphragm 24 of the primary microphone 18A.

[0056] In other embodiments, the diaphragm masses may be different. In
that case, internal or external logic may be used to compensate for the
mass differences. For example, if the mass of the correction microphone
diaphragm 24 is half that of the primary microphone diaphragm 24, then
logic may multiply the signal from the correction microphone 18B by a
scalar value (e.g., a scalar of two). Logic therefore causes the
effective vibration output of the two microphones to be effectively the
same. Stated another way, the mechanical responses of the two microphones
may be considered to be effectively the same if 1) they do, in fact, have
the same diaphragm masses, or 2) if logic compensates for diaphragm mass
differences to effectively cause them to appear the same (e.g., applying
a scalar). In yet other embodiments, the two microphones may be entirely
different and thus, other logic is required to ensure accurate results
consistent with those discussed herein.

[0057]FIG. 3B schematically shows another embodiment in which the gaps
discussed above are substantially identical. Despite having identical
gaps, the correction microphone 18B still is configured to have a
frequency response as shown in FIG. 4B (i.e., having a higher low
frequency cut-off). To that end, the diaphragm 24 of the correction
microphone 18B has one or more perforations or through-holes that
effectively increase the low frequency cut-off. Specifically, the low
frequency cut-off is determined by the amount of area defined by the gap
and the hole(s) through the diaphragm 24. This area thus is selected to
provide the desired cutoff frequency.

[0058] In general terms, the embodiments shown in FIGS. 3A and 3B are two
of a wide variety of means for controlling the air leakage past the
respective diaphragms 24. In other words, those embodiments control the
rate at which air flows past the diaphragm, thus controlling the
respective low frequency cut-off points. Those skilled in the art
therefore can use other techniques for adjusting the desired low
frequency cut-off of either microphone 18A and 18B.

[0059] As noted above, illustrative embodiments combine the correction
signal with the primary signal to remove the noise component from the
primary signal. To that end, FIG. 5 schematically shows various elements
of the microphone system 12 for accomplishing those ends. In general, the
microphone system 12 has a primary microphone apparatus 40 having the
primary microphone 18A, and a correction microphone apparatus 42 having
the correction microphone 18B and two subsequent processing stages 46 and
48 (i.e., logic 38). Summation logic 44 (also referred to as "combining
logic 44") combines the outputs from the two microphone apparatuses to
generate an output audio signal that preferably has a relatively low
noise component.

[0060] As noted above, the correction microphone apparatus 42 generates a
noise component for mitigating the noise component of the primary signal.
To that end, the correction microphone apparatus 42 has 1) a low pass
filter 46 for substantially mitigating audio components in the correction
signal received from the correction microphone 18B, and 2) a signal
transformation module 48 for normalizing the audio response to the
mechanical portion of the noise component.

[0061] More specifically, although they illustratively are very similar,
the two microphones 18A and 18B still may have some differences. For
example, due to the tolerances and limits of their fabrication process,
the microphones 18A and 18B may have some minor differences, such as the
diaphragm thickness. In fact, as noted herein, some embodiments use
different types of devices to serve the function of one or both of the
microphones 18A and 18B (e.g., the correction microphone 18B may be a
conventional accelerometer). As another example, the microphones 18A and
18B are spaced from each other. The correction microphone 18B therefore
may receive a slightly time delayed version of an audio and/or noise
signal.

[0062] Unless normalized, these differences can cause the noise components
of the two microphones 18A and 18B to vary. If they vary too much, the
output signal may be corrupted or have a less desirable signal to noise
ratio. Illustrative embodiments thus compensate for the impact of these
and other differences between the two microphones 18A and 18B to ensure
that the two microphones 18A and 18B have substantially identical noise
components. As noted above, this process may be referred to herein as a
"normalization" process.

[0063] To that end, the signal transformation module 48 compensates for
differences between the primary microphone 18A and the correction
microphone 18B. In illustrative embodiments, the signal transformation
module 48 is a conventional adaptive filter. In alternative embodiments,
the signal transformation module 48 is a fixed filter. Other devices may
be used to achieve the noted results. The respective filters may be any
conventionally known filter used for the noted purposes. For example, if
used, the adaptive filter may be a least mean squared adaptive filter,
also referred to in the art as an "LMS" filter.

[0064] Accordingly, in illustrative embodiments, the correction microphone
apparatus 42 generates a signal having no greater than a negligible
amount of the audio signal, thus substantially comprising a noise
component. It is this noise component that is used to remove the
corresponding noise component generated by the primary microphone
apparatus 40.

[0065] The microphone system 12 therefore has combining logic 44 to
combine the two signals. Among other things, as noted above, the
combining logic 44 may include conventional subtraction logic that
subtracts the signal generated by the correction microphone apparatus 42
from the signal generated by the primary microphone apparatus 40. In
alternative embodiments, the combining logic 44 may include an adder. For
example, in such embodiments, the microphones 18A and 18B may be
positioned within the base 10 to generate signals that are 180 degrees
out of phase. More specifically, it is contemplated that one microphone
could be oriented so that the top surface of its diaphragm 24 faces
upwardly, while the other microphone could be oriented so that the top
surface of its diaphragm 24 faces downwardly. Of course, those skilled in
the art should understand that other combining logic 44 may be used to
facilitate system implementation.

[0066] It should be noted that the signal generated by the correction
microphone apparatus 42 may be considered to be the above noted
correction signal (i.e., as processed by the filters). In a similar
manner, the signal generated by the primary microphone apparatus 40 also
may be considered to be the above noted primary signal (i.e., as
processed by any intervening logic elements, not shown). Accordingly, for
simplicity, the output signals of the primary microphone apparatus 40 and
the correction microphone apparatus 42 respectively are referred to as
the primary signal and correction signal.

[0067]FIG. 6 shows a process of generating an output audio signal in
accordance with illustrative embodiments of the invention. The process
begins at step 600 by substantially mitigating the audio component from
the signal generated by the correction microphone 18B. To that end, the
correction output signal is filtered by the low pass filter 46. As noted
above, when using the embodiments of FIGS. 3A, 3B, or other related
embodiment, the correction microphone 18B naturally filters signals
having frequencies that are less than the low frequency cut-off of the
low pass filter 46. For example, if the frequency response of the
correction microphone 18B has a low frequency cut-off of about 200 Hz,
then the low pass filter 46 should similarly have a high frequency
cut-off of about 190-200 Hz or greater.

[0068] Accordingly, after executing step 600, the correction output signal
should have substantially no non-negligible audio component corresponding
to an input audio signal. This step may be skipped, however, for those
embodiments that shield the diaphragm 24 from the input audio signal.

[0069] Before, contemporaneously with, or after executing step 600, the
process normalizes the audio response to the mechanical portion of the
noise component (step 602). Embodiments that do not remove this audio
response may skip this step. In the general case, however, the signal
transformation module 48 may be retained in the system as an all pass
filter that is selectively activated. Alternatively, the signal
transformation module 48 may be eliminated.

[0070] At this point in the process, the correction microphone apparatus
42 should have generated a correction signal having a noise component
that is substantially identical to the noise component in the primary
signal. Both signals thus are forwarded to the summation logic 44 to
remove/mitigate the noise component from the primary signal (step 604),
thus ending the process. In other words, step 604 removes both the
mechanical portion of the noise signal, as well as its associated audio
response.

[0071] As noted above, depending upon the orientation of the microphones
18A and 18B, the summation logic 44 may subtract or add the two signals.
Of course, other logic may be used in place of, or in addition to, the
discussed subtraction and addition logic. Accordingly, discussion of
specific subtraction or addition logic is illustrative only and not
intended to limit all embodiments of the invention.

[0072] Illustrative embodiments therefore should significantly improve
signal to noise ratios over conventional single microphone systems known
to the inventors. It should be reiterated that various of the components
shown in the drawings are illustrative and not intended to limit the
scope of all embodiments. For example, additional components may be used
to optimize operation. As another example, the microphone system 12 may
have more than two microphones 18A and 18B or microphone apparatuses.
Instead, among other things, the microphone system 12 may have three or
more microphones, or three or more microphone apparatuses. Moreover, one
or more of the microphone apparatuses may include just microphones (e.g.,
the correction microphone 18B may have just a cap 34 with no audio input
port).

[0073] Various embodiments of the invention may at least have portions
implemented at least in part in any conventional computer programming
language. For example, some embodiments may be implemented in a
procedural programming language (e.g., "C"), or in an object oriented
programming language (e.g., "C++"). Other embodiments of the invention
may be implemented as preprogrammed hardware elements (e.g., application
specific integrated circuits, FPGAs, and digital signal processors), or
other related components.

[0074] In an alternative embodiment, some portions of the disclosed
apparatus and methods (e.g., see the flow chart described above) may be
implemented as a computer program product for use with a computer system.
Such implementation may include a series of computer instructions fixed
either on a tangible medium, such as a computer readable medium (e.g., a
diskette, CD-ROM, ROM, or fixed disk) or transmittable to a computer
system, via a modem or other interface device, such as a communications
adapter connected to a network over a medium. The medium may be either a
tangible medium (e.g., optical or analog communications lines) or a
medium implemented with wireless techniques (e.g., WIFI, microwave,
infrared or other transmission techniques). The series of computer
instructions can embody all or part of the functionality previously
described herein with respect to the system.

[0075] Those skilled in the art should appreciate that such computer
instructions can be written in a number of programming languages for use
with many computer architectures or operating systems. Furthermore, such
instructions may be stored in any memory device, such as semiconductor,
magnetic, optical or other memory devices, and may be transmitted using
any communications technology, such as optical, infrared, microwave, or
other transmission technologies.

[0076] Among other ways, such a computer program product may be
distributed as a removable medium with accompanying printed or electronic
documentation (e.g., shrink wrapped software), preloaded with a computer
system (e.g., on system ROM or fixed disk), or distributed from a server
or electronic bulletin board over the network (e.g., the Internet or
World Wide Web). Of course, some embodiments of the invention may be
implemented as a combination of both software (e.g., a computer program
product) and hardware. Still other embodiments of the invention are
implemented as entirely hardware, or entirely software.

[0077] Although the above discussion discloses various exemplary
embodiments of the invention, it should be apparent that those skilled in
the art can make various modifications that will achieve some of the
advantages of the invention without departing from the true scope of the
invention.